1. Field of the Invention
The present invention relates to a method for producing polyamides and to a method for producing a primary polycondensate which is an intermediate for polyamide production. More precisely, it relates to a method for producing a primary polycondensate, which comprises polycondensing a specific dicarboxylic acid component and a specific diamine component in the presence of a predetermined amount of water at a predetermined reaction temperature and under a predetermined reaction pressure to give a primary polycondensate, followed by taking the primary polycondensate out of the reactor to be in an atmospheric environment while the temperature and the water content of the primary polycondensate are still within the same ranges as those in the previous polycondensation step. The invention also relates to a method for producing polyamides, which comprises further polymerizing the primary polycondensate into a polyamide having an increased molecular weight.
2. Description of the Background
Since they have excellent properties and good melt-moldability, crystalline polyamides such as typically nylon 6, nylon 66 and others, have been being used widely for clothing, fibers for industrial materials, engineering plastics, etc. On the other hand, however, these general-purpose polyamides are regarded as problematic in that their heat resistance is poor and, when they have absorbed water, they often lose dimensional stability.
In recent years, in particular, high-quality polyamides in the field of electrical and electronic components, car parts, engineering plastics, etc., have been needed. For, example, with the development in the surface mounting technique (SMT) in the field of electrical and electronic devices, the polyamides to be used are required to have high heat resistance including reflow soldering heat resistance. Also, for car parts such as engine room parts, needed are polyamides having much more improved heat resistance. With their applications expanding, polyamides are being used not only in the field of electrical and electronic components and car parts but also in other various fields in which are needed polyamides having much better physical properties and functions. Given that situation, it is necessary to develop high-quality polyamides having not only good heat resistance but also good dimensional stability, good mechanical properties and good chemical resistance. In addition, it is also necessary that polyamides are easy to handle while they are produced through polymerization and while they are molded and worked into articles.
To meet the requirements, (1) a method of-producing semi-aromatic polyamides from a dicarboxylic acid component essentially consisting of terephthalic acid and isophthalic acid or adipic acid and a diamine component essentially consisting of 1,6-hexanediamine or the like (see JP-A61-228022, 3-72564, 8-59825, 8-198963, etc.), and (2) a method of producing polyamides from 1,4-butanediamine and adipic acid (see U.S. Pat. No. 4,722,997) as known, and some of them have been industrialized.
For producing polyamides, a batch process of directly polymerizing a dicarboxylic acid component and a diamine component in melt in a pressure reactor has heretofore been widely employed. In this process, the polyamide produced is taken out of the reactor while in melt form. In the process, however, the polyamide produced must be kept at high temperatures of not lower than its melting point for a long period of time in the latter stage of the reaction and while the polyamide is taken out of the reactor. As a result, the polyamide is often degraded by heat, and its quality is degraded.
In particular, the semi-aromatic polyamides obtained according to technique (1) and the polyamides produced from 1,4-butanediamine and adipic acid according to technique (2) are readily pyrolyzed in the conventional batch process of direct melt polymerization, since their melting point is close to their decomposition point. In techniques (1) and (2), therefore, it is difficult to increase the molecular weight of polyamides without pyrolysis thereof.
Therefore, in techniques (1) and (2), monomers are not subjected to direct melt polymerization to give the desired polyamides. In melt polymerization, monomers are first condensed into a low-order condensate (primary condensate), and the low-order condensate is further polymerized into the desired polyamides having an increased molecular weight.
However, in technique (1), when 1,6-hexanediamine is used as the diamine component, the amido-group concentration in the polymer produced is increased. The chemical resistance, the water absorption resistance and the melt stability of the polymer having such a high amido-group concentration are poor. In addition, in (1), the dicarboxylic acid component comprises isophthalic acid and/or adipic acid as the comonomer, in addition to terephthalic acid, and the amount of the comonomer is relatively large. Copolymerization with isophthalic acid lowers not only the degree of crystallinity of the polymer formed but also the heat resistance, the chemical resistance, the water absorption resistance and the dimensional stability of the polymer. Copolymerization with adipic acid lowers the heat resistance and the melt stability of the polymer.
The polyamides obtained according to technique (2) are poly (tetramethyleneadipamide), which are of a type of aliphatic polyamides, and the heat resistance, chemical resistance and water absorption resistance thereof are inferior.
Regarding technique (1), the method described in JP-A 61-228022 and 3-72564 is problematic in that the low-order condensate formed therein has a low limiting viscosity and therefore could not be directly subjected to solid-phase polymerization. Therefore, in the method disclosed, the low-order condensate formed is once polymerized in melt into a prepolymer, and thereafter the resulting prepolymer is subjected to solid-phase polymerization into the intended semi-aromatic polyamides. The method comprises the multi-stage polymerization. This method requires complicated production steps and complicated equipment. Accordingly, the method requires significant labor and is expensive. In addition, in this method, an additional pressure container in which the pressure is controlled to be a predetermined one is provided adjacent to the outlet of the polymerization reactor, and the low-order, condensate produced in the reactor is taken out into the pressure container. This is in order to reduce the pressure difference between the reactor and the collector into which the low-order condensate is to be taken out of the reactor (see example 1 in JP-A3-72564). Therefore, the method requires the special pressure container having a specifically controlled inner pressure. The pressure container of that type requires special process control and equipment, and the method is complex and expensive.
Regarding technique (1), the method described in JP-A 8-59825 is problematic in that the step of preparing the primary condensate from starting monomers of essentially terephthalic acid and adipic acid, and 1,6-hexanediamine is actually effected at a high temperature above 280.degree. C., and therefore, the primary condensate prepared is readily degraded under heat. In addition, in the method described therein, the pressure under which the primary condensate is prepared is low, specifically, it is lower than 23 kg/cm.sup.2 G. Under such low pressure, a large amount of the monomers being reacted to give the primary condensate vaporize away during the reaction with the result that the proportions of the monomer, units constituting the resulting primary condensate significantly differ from those of the starting monomers as fed into the reactor. In this method, therefore, the primary condensate prepared often loses the original molar balance of the starting monomers.
Regarding technique (1), the method described in JP-A 8-198963 is problematic in that the primary condensate formed from starting monomers of essentially terephthalic acid and/or adipic acid, and 1,6-hexanediamine and/or dodecamethylenediamine is too much foamed (the foaming magnification is 5-fold or more), and has a low bulk density. Therefore in the post-step of polymerizing the primary condensate into a polymer having an increased molecular weight, grains of the primary condensate are readily broken or they often adhere to the wall of the polymerization reactor. In the post-polymerization step, the primary condensate grains are difficult to handle, and, in addition, the volume efficiency in the polymerization reactor is low.
In the method described in JP-A 8-59825 and 8-198963, the primary condensate formed is taken out of the reactor while water is fed thereinto through a separate line. In the method, therefore, taking out the primary condensate from the reactor involves time-consuming and troublesome operation.
Accordingly, there remains a need for methods of producing polyamides which overcome the disadvantages discussed above.